The G Protein β Subunit Is a Determinant in the Coupling of Gs to the β1-Adrenergic and A2a Adenosine Receptors

The signaling specificity of five purified G protein βγ dimers, β 1 γ 2 , β 2 γ 2 , β 3 γ 2 , β 4 γ 2 , and β 5 γ 2 , was explored by reconstituting them with G s α and receptors or effectors in the adenylyl cyclase cascade. The ability of the five βγ dimers to support receptor-α-βγ interactions was examined using membranes expressing the β 1 -adrenergic or A2a adenosine receptors. These receptors discriminated among the defined heterotrimers based solely on the β isoform. The β 4 γ 2 dimer demonstrated the highest coupling efficiency to either receptor. The β 5 γ 2 dimer coupled poorly to each receptor, with EC 50 values 40–200-fold higher than those observed with β 4 γ 2 . Strikingly, whereas the EC 50 of the β 1 γ 2 dimer at the β 1 -adrenergic receptor was similar to β 4 γ 2 , its EC 50 was 20-fold higher at the A2a adenosine receptor. Inhibition of adenylyl cyclase type I (AC1) and stimulation of type II (AC2) by the βγ dimers were measured. βγ dimers containing Gβ 1–4 were able to stimulate AC2 similarly, and β 5 γ 2 was much less potent. β 1 γ 2 , β 2 γ 2 , and β 4 γ 2 inhibited AC1 equally; β 3 γ 2 was 10-fold less effective, and β 5 γ 2 had no effect. These data argue that the β isoform in the βγ dimer can determine the specificity of signaling at both receptors and effectors.